1. Field of the Invention
An aspect of the present invention relates to a liquid-gas separator for segregating carbon dioxide and an unreacted liquid fuel discharged from an anode electrode of a direct liquid feed fuel cell.
2. Description of the Related Art
A direct liquid feed fuel cell is an apparatus that generates electricity by electrochemical reactions between an organic fuel, such as methanol or ethanol, and an oxidant, i.e., oxygen. The electricity generated by the direct liquid feed fuel cell has a high specific energy density and a high power density. Also, since liquid fuel, i.e., methanol, is fed directly to the cell, the direct feed fuel cell does not require a peripheral device, such as a fuel reformer, and storing and supplying the liquid fuel are easy.
As depicted in FIG. 1, the direct feed fuel cell has a structure including an anode electrode 2, a cathode electrode 3, and an electrolyte membrane 1 interposed between the two electrodes 2 and 3. The anode electrode 2 includes a diffusion layer 22 for supplying and diffusing fuel, a catalyst layer 21 at which oxidation reaction of the fuel occurs, and an electrode supporting layer 23. The cathode electrode 3 also includes a diffusion layer 32 for supplying and diffusing the fuel, a catalyst layer 31 at which reduction reaction occurs, and an electrode supporting layer 33. The catalyst for generating the electrode reaction is formed of a precious metal, such as platinum, having superior catalytic characteristics at low temperature. Alternately, to avoid catalyst poisoning by CO, which is a by-product of the electrode reaction, a transition metal alloy catalyst comprising ruthenium, rhodium, osmium, or nickel can be used. The electrode supporting layers 23 and 33 can be made of waterproofed carbon paper or waterproofed carbon fiber for easily supplying fuel and discharging reaction products. The electrolyte membrane 1 is a hydrogen ion exchange membrane having ion conductivity and containing moisture, and is formed of a polymer membrane having a thickness of 50˜200 μm.
An electrode reaction of a direct methanol fuel cell (DMFC), which is a type of direct liquid feed fuel cell, includes an anode reaction where fuel is oxidized and a cathode reaction where hydrogen and oxygen are reduced, as described below.CH3OH+H2O→CO2+6H++6e− (Anode reaction)  [Reaction 1]3/2 O2+6H++6e−→3H2O (Cathode reaction)  [Reaction 2]CH3OH+3/2 O2→2H2O+CO2 (Overall reaction)  [Reaction 3]
Carbon dioxide, hydrogen ions, and electrons are produced at the anode electrode 2 where the fuel is oxidized (reaction 1). The hydrogen ions migrate to the cathode electrode 3 through a hydrogen ion exchange membrane 1. Water is produced by the reduction reaction between hydrogen ions, electrons transferred from an external circuit, and oxygen at the cathode electrode 3 (reaction 2). Accordingly, water and carbon dioxide are produced as the result of an overall electrochemical reaction (reaction 3) between methanol and oxygen. Two moles of water are produced when one mole of methanol reacts with oxygen.
The liquid fuel used in the fuel cell may not be pure methanol, but may be a mixture with water produced in the system or already stored in the fuel cell system. When a fuel of high concentration is used, the performance of the fuel cell is greatly reduced due to crossover of the fuel through the electrolyte membrane (hydrogen ion exchange membrane). Therefore, methanol diluted to a low concentration, such as 0.5 to 2 M (2 to 8 volume %), is generally used.
FIGS. 2A and 2B are cross-sectional views of a liquid-gas separator used for a fuel cell. The orientation of the liquid-gas separator 10 used for a mobile fuel cell is not fixed at one orientation. At a normal orientation (refer to FIG. 2A), unreacted fuel and carbon dioxide enter the liquid-gas separator 10 through an inlet 11. Carbon dioxide is exhausted into the air through a hole 12 formed on a ceiling of the liquid-gas separator body, and the unreacted fuel is recovered to the fuel cell through an outlet 13 formed on a lower part of the liquid-gas separator body.
However, at a reversed orientation (refer to FIG. 2B) of the liquid-gas separator 10, the outlets 12 and 13 of the unreacted fuel and carbon dioxide are changed. Accordingly, the carbon dioxide may pass into the anode electrode, and the unreacted fuel can be discharged to the outside.